Cystic fibrosis (CF) is a lethal, recessive, genetic disease affecting approximately 1 in 2500 live births among Caucasians. (Cohen-Cymberknoh M, Shoseyov D, Kerem E. Managing cystic fibrosis: strategies that increase life expectancy and improve quality of life. Am J Respir Crit Care Med (2011); 183: 1463-1471; Boat T F, Welsh M J and Beaudet A L. Cystic fibrosis. (1989) IN “The Metabolic Basis of Inherited Disease” (C L Scriver, A L Beaudet, W S Sly and D Valee, eds.), 6th Ed., pp. 2649-2680. McGraw-Hill, New York). Approximately 1 in 25 persons are carriers of the genetic defect associated with the disease. The major symptoms of cystic fibrosis include chronic pulmonary disease, pancreatic exocrine insufficiency, infertility in males, and elevated sweat electrolyte levels. The symptoms are consistent with cystic fibrosis being an exocrine disorder. (Hantash F: U.S. Patent Application No. 20060057593. Method for detecting cystic fibrosis. (2004). Published Mar. 16, 2006).
The CF gene codes for a cAMP/PKA-dependent, ATP-requiring, membrane-bound chloride ion channel known as CFTR (cystic fibrosis transmembrane conductance regulator), and is generally localized to the apical membranes of many secreting epithelia There are currently over 1700 known mutations affecting CFTR, many of which give rise to a disease phenotype. Around 75% of CF alleles contain the ⇄F508 mutation in which a triplet codon has been lost, leading to a missing phenylalanine at position 508 in the protein. This altered protein fails to be trafficked to the correct location in the cell and is generally destroyed by the proteasome. The small amount that does reach the correct location functions poorly. (Cutbert A W. New horizons in the treatment of cystic fibrosis. British J Pharm, (2011), 163: 173-183).
Although CFTR functions mainly as a chloride channel, it has many other roles, including inhibition of sodium transport through the epithelial sodium channel, regulation of the outwardly rectifying chloride channel, ATP channels, intracellular vesicle transport, and inhibition of endogenous calcium-activated chloride channels. CFTR is also involved in bicarbonate-chloride exchange. A deficiency in bicarbonate secretion leads to poor solubility and aggregation of luminal mucins. Obstruction of intrapancreatic ducts with thickened secretions causes autolysis of pancreatic tissue with replacement of the body of the pancreas with fat, leading to pancreatic insufficiency with subsequent malnutrition. In the lungs, CFTR dysfunction leads to airway surface liquid (ASL) depletion and thickened and viscous mucus that adheres to airway surfaces. The result is decreased mucociliary clearance (MCC) and impaired host defenses. Dehydrated, thickened secretions lead to endobronchial infection with a limited spectrum of distinctive bacteria, mainly Staphylococcus aureus and Pseudomonas aeruginosa, Deficiency in bicarbonate secretion due to loss of CFTR function also results in a lower pH at the airway surface which impairs anti-bacterial killing activity and increases susceptibility to infection. An exaggerated inflammatory response in response to chronic lung infections leads to the development of bronchiectasis and progressive obstructive airways disease. Pulmonary insufficiency is responsible for most CF-related deaths. (Cohen-Cymberknoh M, Shoseyov D, Kerem E. Managing cystic fibrosis: strategies that increase life expectancy and improve quality of life. Am J Respir Crit Care Med (2011); 183: 1463-1471).
The prognosis for the treatment of CF has improved over the last 40 years. This was achieved by improving pancreatic enzyme supplements, drugs designed to treat pulmonary infection, reduce inflammation and enhance mucociliary clearance. Currently the therapeutic challenges are to correct the biochemical defect of CF and to identify effective treatments for chronic respiratory infection. (Frerichs C, Smyth A. Treatment strategies for cystic fibrosis: what's in the pipeline? Pharmacotherapy (2009), 10: 1191-1202).